The present invention relates generally to a new apparatus and insertless method for forming cavities in semiconductor substrates.
Semiconductor substrates and devices are becoming smaller and more dense with the evolution of new technologies. However, increases in circuit density produce a corresponding increase in overall manufacturing problems. These manufacturing problems must however be kept to a minimum in order for the semiconductor manufacturer to remain competitive. The semiconductor manufacturers are therefore constantly being challenged to improve the quality of their products by identifying and eliminating defects which produce defective parts or components. Whereas significant improvements are being made to eliminate systematic defects by reducing process variability, process improvements alone are not sufficient to eliminate all the random defects which effect both yield and reliability. Historically, screening techniques have been employed to improve product failure rates to acceptable levels by culling out many of these random defects.
In their desire to improve their products, the semiconductor manufacturers are constantly finding new ways and new techniques to improve or provide new products. It has been found that for some applications, one-could make a ceramic carrier or substrate (hereafter just substrate) having a cavity and then have the semiconductor chip placed inside the cavity and secured to the substrate. These semiconductor substrates are often referred to as modules and could be made from a single ceramic layer or green sheet forming a single layer ceramic module or a plurality of ceramic layers forming an MLC (multilayer ceramic) module.
While the remaining discussion will be directed to MLC modules, it should be understood that the teachings of the present invention can be equally applicable to single layer modules.
MLC modules having single or multiple cavities are normally used in the electronic industry to package high performance integrated circuits or chips (hereafter just chips). These high performance chips have a large number of external inputs/outputs (called I/Os), such as pads or solder balls, to name a few, and these chips have a very high power dissipation. In order to accommodate such high performance chips, the MLC module also has to provide a high number of external I/Os, such as pads, pins, solder balls, to name a few, and also be able to handle the very high power dissipation that is being generated both from the module as well as the chip. Further, there may also be wire bond pads on the shelves in the cavity.
The single or multiple cavities in an MLC substrate are normally laminated during the lamination process typically with the aid of a hard or soft insert as a plug such as that disclosed in Phillips, IBM Technical Disclosure Bulletin, April 1974, page 3559, the disclosure of which is incorporated by reference herein. This insert in turn prevents the collapse or deformation of the stacked green ceramic body during lamination. This method of producing single or multiple cavities requires machining of the inserts with high precision and with high level of surface finish.
Inherently, the cost of such inserts is very high compared to the cost of the substrate. Additionally, these inserts or plugs do not provide the flexibility of using the same inserts for cavities of various shapes and sizes. Furthermore, placing these inserts in the cavities and then subsequently removing them is an expensive process and many times removing them could lead to the delamination of the ceramic green sheets or other damage to the green ceramic body. Another drawback with these solid inserts is the need to clean them prior to every use to avoid the paste pull-outs or damage to the green ceramic layers or pads. Even with cleaning, paste pull-out often occurs due to the lack of an effective release layer.
Another method of producing these single or multiple cavities in the MLC substrate would be to machine the cavities after the green sheets have been stacked and laminated, but this would not be a cost effective way of producing parts in a high volume manufacturing operation.
It is also possible to form cavities in the MLC substrate with no inserts. This could be done for cases where the lamination conditions are such that there is no resulting deformation in the green ceramic body. In these cases, typically, the lamination pressures are very low and the green sheet formulation is such that the dimensional control of products is achieved by altering the sintering process. However, in a high volume manufacturing operation, tailoring the green sheet formulation and developing a sintering cycle for every product would be cost prohibitive and time consuming. Besides, this approach typically needs an adhesive between layers and multiple lamination steps to achieve the end result. Thus, some of the problems associated with this low pressure lamination process are that no process window for dimensional control is available for the sintered body. Delamination of the ceramic greensheets could happen in sintering due to the removal of the adhesive and the density gradients in the starting structure that are normally present could result in poor substrate dimensional control. Furthermore, there could be substantial increase in stacking and lamination cost and limitation in metal loading on the green sheets to have effective green sheet bonding.
The prior art has approached this problem in other ways as well.
McNeal et al. U.S. Pat. No. 4,680,075 and Norell U.S. Pat. No. 4,636,275, the disclosures of which are incorporated by reference herein, disclose laminating with a preformed thermoplastic or rubber plug or with a rubber bladder. A low durometer rubber (one with a durometer rating of less than Shore A 30) may be used. During the actual lamination process, no release layer or coating was disclosed although a release layer of Tedlar is suggested during the formation of the preformed plug.
Bloechle et al. U.S. Pat. No. 4,737,208, the disclosure of which is incorporated by reference herein, discloses a method of laminating a printed wiring board (non-ceramic) by utilizing a metallic template (which relieves some of the stresses on the corners of the layers), a fluropolymer (such as TFE) release layer and a conformal material (for example, butyl rubber) which fills the cavity.
In every cavity formation technique, it is essential that the material set is chosen such that a cavity profile with sharp corners and flat wire bonding shelves is achieved. When improper release and/or membrane materials are chosen, rounded edges and corners, sloped wire bond shelves and paste pullout are the result.
The present invention, however, solves these problems as described more fully in the following description taken with the accompanying drawings.
The invention is a novel apparatus and insertless method for forming cavity substrates using a coated membrane sheet.
Therefore, one purpose of the present invention is to provide an apparatus and method that will form cavities in a semiconductor substrate without the necessity of using an insert.
Another purpose of the present invention is to provide an apparatus and method that will form cavities in a semiconductor substrate without causing damage to the ceramic body or paste pull-outs.
Yet another purpose of the present invention is the providing of a coated membrane sheet which is essential to the avoiding of damage to the ceramic body and paste pull-outs.
Therefore, one aspect of the invention relates to a method of forming a ceramic substrate having at least one cavity, the method comprising the steps of:
a) placing at least one ceramic greensheet having at least one cavity over a first plate;
b) placing a planar coated membrane sheet over said cavity, wherein said membrane sheet has an elongation of greater than 350% at room temperature and a modulus of less than 1 GPa.;
c) conforming said coated membrane sheet to said cavity;
d) applying pressure to at least the coated membrane sheet and the at least one ceramic greensheet, said coated membrane sheet preventing the collapse of said cavity during this step of applying pressure.
A second aspect of the invention relates to an apparatus for forming cavities in at least one ceramic green sheet of a ceramic substrate comprising:
a.) a first plate to support said at least one greensheet, said at least one green sheet having a cavity;
b.) a planar coated membrane sheet placed over said at least one green sheet and said cavity, wherein said membrane sheet has an elongation of greater than 350% at room temperature and a modulus of less than 1 GPa.;
wherein, in operation, a pressure is applied to cause said planar coated membrane sheet to conform to the contours of the cavity and a further pressure is applied against said at least one ceramic layer, said coated membrane sheet preventing the collapse of the cavity during the application of said further pressure.